This invention relates to a method for siliconization of carbon-containing materials and a device adapted to this purpose.
A method for siliconization of carbon-containing materials was described by a working group of the DLR (Deutsche Forschungsanstalt für Luft-und Raumfahrt e.V. [German Aerospace Center]), Stuttgart, Institute of Structures and Design, at the VDI [Association of German Engineers] Materials Conference in Duisburg from Mar. 9 to 10, 1994. Carbon materials reinforced with carbon fibers were infiltrated with molten silicon by means of a so-called “liquid siliconization process.” The elementary silicon reacts with carbon to form silicon carbide by heat treatment. Structural components and in particular also friction bodies for brake and clutch systems in motor vehicles can be produced from these materials.
Similar methods are described, for example, in EP 0 956 276, the carbon workpieces to be siliconized and suitably bound powdered silicon being heated together, the molten silicon penetrating into the carbon workpieces and reacting with the carbon at least partially to form silicon carbide.
The described methods are batch methods in which conversion of carbon materials into materials with a silicon carbide-containing matrix takes place batch by batch. In this case the furnaces in which the silicon is melted and in which the reaction of liquid silicon with carbon into silicon carbide is effected, must first be charged, heated under an inert gas atmosphere and optionally evacuated, and after the reaction to silicon carbide must be cooled with the holding time necessary for this purpose and unloaded. These methods require long heating and cooling times and are unfavorable with respect to energy use.
A favorable method therefore comprises treatment of workpieces of porous carbon which is optionally reinforced with fibers, preferably carbon fibers, with liquid silicon, which treatment can be carried out continuously or semicontinuously in a cyclic manner, with at least partial conversion of the carbon into silicon carbide. Such a continuous or semicontinuous method for siliconization is described in application DE 10 2006 009 388.7. But the siliconizing step is discontinuous, the body to be impregnated is seated on wicks, and is not transported during silicon infiltration. The delivery of silicon is also nonuniformly distributed by the wicks over the support surface; this can lead to inhomogeneity of the impregnated body. Therefore the object is to devise a continuous method also for the step of siliconization, that is, delivery of liquid silicon to workpieces of porous carbon which have optionally been reinforced with fibers, preferably those of carbon, and the subsequent reaction of the silicon which has been taken into the workpieces with at least part of the carbon in the workpieces to silicon carbide.                The object is achieved by a process which comprises the following steps:a) preheating of porous carbon workpieces reinforced optionally with fibers under an inert gas, proceeding from room temperature to a selected operating temperature TB1,c) delivery of liquid silicon to the porous carbon workpieces at an operating pressure pB2 and an operating temperature TB2 and impregnation of the porous carbon workpieces with liquid silicon,d) reaction of the liquid silicon which has penetrated into the workpieces with carbon in the workpieces at a temperature TB3 with formation of silicon carbide,e) gassing of the workpieces with inert gas and cooling from the operating temperature TB3 to a conditioning temperature Tk with continuation of the reaction and reduction of the stresses formed in the workpieces,f) cooling of workpieces to room temperature,in step c the delivery of silicon and transport of the workpieces taking place over preferably cylindrical rolls which are porous at least in the exterior region and which are pivoted, and their speed of rotation determining the residence time for the delivery of silicon in step c and the initial reaction of the silicon with the carbon with the formation of silicon carbide, and the temperature TB3 being greater than or equal to the temperature TB2, and the workpieces for process step d no longer being in contact with liquid silicon outside the workpieces.        